1. Field of the Invention
The present invention is generally concerned with a variable flow cryostat and is particularly concerned with a cryostat having a plastic sensor which maintains a stable refrigerant flow rate over a wide range of operating conditions and which may be selectively exposed to fluid refrigerant for controlling the throttling sensitivity of the cryostat.
1. Description of Prior Developments
Cryostats have been frequently used to maintain a particular cold operating temperature within an enclosure such as a dewar flask by controlling the release of a refrigerant into the enclosure. A demand flow cryostat, which varies the release of fluid refrigerant based on the heat load and operating temperature in the enclosure, typically includes a refrigerant supply connection in the form of an inlet tube, a heat exchanger such as a finned coiled tube, and a temperature-controlled variable flow nozzle or orifice.
The refrigerant is usually supplied to the cryostat as a high pressure gas at the local ambient temperature. Initially, the cryostat itself is also typically at the same local ambient temperature. The high pressure refrigerant gas flows through the inlet tube to the cryostat and then through the inside of the heat exchanger tube.
The heat exchanger tube generally has an external extended surface, such as fins, to carry the heat radially away from the inside surface of the tube. The high pressure gas then exits through the orifice of the variable flow nozzle and flows into a lower pressure area contained by the dewar. The lower pressure generally coincides with the local ambient pressure.
The pressure drop in the gas flowing through the orifice occurs supersonically and isenthalpically so that the gas experiences a temperature reduction relative to its pressure reduction and in accordance with the well known Joule-Thomson principle. The lower pressure and now lower temperature gas, which is contained by the dewar, is forced to flow over the external extended surface on the heat exchanger. This colder exhaust gas flow is counter to the direction of the warmer incoming gas flow so that heat is exchanged.
The warmer higher pressure incoming gas is cooled while the colder lower pressure exhaust gas is warmed. The exhaust gas flows toward the warm end exit of the heat exchanger and into the local ambient region. Eventually, as flow continues at an appropriate rate, the high pressure gas that exits through the orifice is pre-cooled enough during its flow through the heat exchanger to form vapor and liquid at the lower pressure.
This process continues during the cooldown period until an equilibrium is established. The heat from the cryostat, sensor assembly, and dewar must be removed before equilibrium or steady-state operation is achieved. Throttling occurs usually prior to such equilibrium.
The cold low pressure liquid refrigerant that is created during expansion through the orifice is contained by the dewar at the cold end of the cryostat. The cold, turbulent, low pressure liquid absorbs heat depending on the heat source and load. This causes the liquid to boil, vaporize and then convert into gas again as it continues to absorb heat while flowing along the heat exchanger toward the warm end exit. This process continues until the supply of high pressure gas is shut off at the inlet to the cryostat.
When inserted into a dewar flask, and operating on the Joule-Thomson principle to provide refrigeration, the demand flow cryostat throttles the flow of refrigerant through the variable flow nozzle. This throttling will occur during or after the cooldown of the cryostat and dewar flask in order to reduce the rate of refrigerant consumption. After the cooldown, the flow of refrigerant is regulated to maintain a stable cold operating temperature in the dewar flask. The throttling rate and flow regulation are generally dependent upon changes in various parameters which govern the action of the temperature-controlled variable flow nozzle.
The fluid refrigerant, typically a pressurized gas or liquid, may include a single component or multiple components. In any case, the cryostat will provide a unique cold temperature at its nominal operating condition and it will have a unique heat exchanger temperature gradient along its length from the warmer ambient temperature end to the colder refrigerated end. The colder end of the cryostat is that end through which the refrigerant issues from the variable flow nozzle.
The heat exchanger temperature gradient is the temperature profile which is established lengthwise and widthwise along the heat exchanger during and after cooldown. The temperature gradient is primarily a function of the refrigerant being used, its supply ambient temperature and pressure, the flow rate, and the heat load applied to the cryostat from the dewar. The temperature at a given position in the heat exchanger is established by its operating with influence from such variables.
Various refrigerant throttling mechanisms of the type adaptable for use with cryostats include bi-metallic mechanisms, gas-charged bellows mechanisms, and bi-material mechanisms. Such devices are, for example, disclosed in U.S. Pat. No. 3,320,755, 3,728,868, and 4,152,903. These throttle mechanisms are typically adjusted to respond in correlation with the temperature of the refrigerant at the cold end of the cryostat or with the heat exchanger temperature gradient which results from the refrigeration process during or after cooldown. These throttle mechanisms can also be adjusted to provide a desired flow rate after cooldown.
The heat that is transferred from the throttle mechanism to the colder refrigerant and heat exchanger causes the throttle mechanism to either expand or contract. This temperature-induced throttle movement is employed to operate a valve or nozzle to reduce or throttle the flow of refrigerant as the temperature surrounding the throttle mechanism approaches a desired operating temperature. After achieving the desired operating temperature, the same temperature sensitive movement is employed to regulate the flow of refrigerant in order to stabilize the operating temperature under conditions of varying refrigerant supply pressure and temperature.
A problem associated with conventional throttle mechanisms is their high sensitivity to temperature gradients. Their tendency is to throttle differently with different refrigerants having different cold end temperatures or causing different heat exchanger temperature gradients. This high sensitivity makes the throttling or regulation of refrigerant flow difficult to control.
Depending on the type of throttle mechanism, its adjusted and set parameters, and the cryostat's operating condition, the refrigerant flow may be unpredictably throttled. That is, throttling may occur either prematurely or incompletely when a refrigerant, having either a warmer or colder cold end temperature or heat exchanger temperature gradient compared to another refrigerant, is supplied to the cryostat. This can lead to unpredictable or undesirable cooldown or post-cooldown operation.
Accordingly, a need exists for a mechanism to control the throttle sensitivity of a temperature-controlled variable flow nozzle of a demand flow cryostat so that the cryostat may be operated reliably with different refrigerants or with different heat exchanger temperature gradients over a wide range of operating conditions.
A further need exists for a method of controlling the thermal resistance and the heat transfer between the throttle mechanism and the refrigerant.
Still another need exists for a cryostat which may be designed with a compact profile for use in applications where space is limited and/or weight is to be minimized.
Yet another need exists for a cryostat which may provide selective or variable access to a throttle mechanism by the cryostat refrigerant.